function add(x: string, y: string): string
function add(x: number, y: number): number
function add(x: string | number, y: string | number): string | number | void{
    if (typeof x === 'string' && typeof y === 'string') {
        return x + y
    }
    if (typeof x === 'number' && typeof y === 'number') {
        return x + y
    }
}
